High-purity and high-quality germanium single crystals are an essential part of crucial devices, such as solar panels for space applications. One of the unsolved problems in the production process of these crystals, is the appearance of micrometer sized voids. When wafers are cut from the crystal, and the wafer contains a void at the surface, the device built on top of it might either be less efficient or not working at all. If the atomistic mechanisms that lead to the growth of voids would be understood, it would be easier to design strategies that can suppress or control their appearance. Important information in that respect might be hiding in the shape of the voids, and in the relation between the shape and the crystal faces that form the inner walls. Experimental information about this can be compared to first principles prediction of the void shape. Such a comparison can learn whether or not the voids have a thermodynamic equilibrium shape or not.

Improved understanding of the issue is valuable information for the semiconductor industry. This research topic will be conducted in the framework of a strong international network and if possible the student will be actively involved in work discussions with collaborative partners.

Goal

This work has an experimental and a computational aspect. It depends on the preference of the candidate whether emphasis will be on the experimental part, the computational part, or on the dialogue between both. The goal of the experimental part is to improve the visualization of the form of the voids. Nowadays the largest voids can be observed as pits on the surface of a polished wafer (see fig.) from which an estimate of their form is not so straightforward. These surface pits can be enlarged using novel preferential etchants allowing a systematic study via optical microscopy. In the computational part, surface energies of several high-index surfaces of germanium will be calculated. These energies are used in a so-called Wulff-construction, in order to obtain the thermodynamical equilibrium shape of a nano-particle (fig) or – in this case – a void. If temperature-dependent surface energies are computed, then the shape of the void can even be predicted as a function of temperature.